1. Field of the Invention
The present invention relates to an optical modulator and an optical modulation method.
2. Description of the Related Art
As a phase modulator used for high-speed optical communications, a Mach-Zehnder modulator has been commercially available. Modulators made of ferroelectric such as lithium niobate are conventionally used as the Mach-Zehnder modulators (for example, see Japanese Patent Application Laid-open No. 2009-171634). Furthermore, semiconductor Mach-Zehnder modulators made of semiconductor are increasingly put to practical use for downsizing and power saving of devices (for example, see “2010, the transactions of the IEICE society conference”, Electronics C-3-54, 2010).
For example, the semiconductor Mach-Zehnder modulator is configured as follows. A first modulation waveguide constitutes a first arm and a second modulation waveguide constitutes a second arm. A demultiplexer splits a light output from a light source and inputs split lights to the first and second arms, respectively. The first and second arms modulate phases of the input lights and a multiplexer combines the phase-modulated lights and outputs a resultant light.
Drivers amplify differential modulation signals generated based on codes to be transmitted from a signal source. First and second bias tees apply biases to the amplified signals and resultant signals are input to the first and second modulation waveguides as voltages, respectively. Waveguide refraction factors of the first and second modulation waveguides change according to the input voltages and the first and second modulation waveguides modulate the phases of lights passing therethrough. A gain controller controls the gain of the driver and voltages applied from a modulator bias controller controls the biases applied by the first and second bias tees.
An operation for performing phase modulation using a semiconductor Mach-Zehnder modulator described above is described below. The output light from the multiplexer is the sum of amplitudes of the lights input to the first and second arms. The gain controller adjusts gains of the drivers so that the lights modulated by the first and second arms have a phase variation π. The gain controller also adjusts the gains of the drivers so that ranges of phase changes by the first and second modulation waveguides shift from each other by π, and so that the phases output from the first and second modulation waveguides become opposite. For example, when the range of the phase change by the first modulation waveguide is 0 to π, the range of the phase change by the second modulation waveguide is π to 2π. With this adjustment, the output light from the multiplexer has two states (for example, 0 and π) at the same amplitude and having a phase difference π. These two states are made to correspond to codes [0] and [1], respectively, thereby performing light transmission by phase modulation.
However, the conventional semiconductor Mach-Zehnder modulator described above has the following problems. In the conventional semiconductor Mach-Zehnder modulator, because phase modulation is performed based on the quantum-confined Stark effect, the first and second modulation waveguides change in absorbed amounts simultaneously with the changes in refraction factors according to the voltages of the differential modulation signals. Therefore, the lights passing through the first and second modulation waveguides change in amplitude simultaneously with the phase changes and the output light changes in amplitude between the codes [0] and [1].